OBJECTIVE To determine whether periaqueductal gray (PAG) neurons that project to the nucleus retroambiguus (NRA) are immunoreactive for estrogen receptor-alpha. In three ovariectomized female monkeys, injections with cholera toxin subunit b (CTb) were made into the NRA to retrogradely label neurons in the periaqueductal gray (PAG). Using immunohistochemistry, estrogen receptors (ER)-alpha containing neurons were identified in the same tissue. The results indicate that the PAG contains large numbers of neurons that project to the NRA and numerous ER-alpha immunoreactive neurons. Although the distribution pattern partly overlaps, only a small number of neurons were double labeled for CTb and ER-alpha. These neurons were mainly present in the ventrolateral PAG. FUTURE DIRECTIONS The data needs to be plotted and quantified. KEY WORDS nucleus retroambiguus, periaqueuductal gray, estrogen receptor-alpha. FUNDING Supplemental funding to RR00167